Cu is known as one of the most promising metallic catalysts for conversion of CO2 to hydrocarbons such as methane, ethylene, and ethanol by electrochemical reduction. The oxide-derived Cu (OD-Cu) moiety has been investigated as a candidate for enhancing the activity for CO2 electrochemical reduction to C2+ products. The reduction process is affected by catalytic grain boundary, local pH, residual oxygen atoms, and other features of the catalysts. In order to understand the detailed mechanism, we performed in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (in situ ATR-SEIRAS) measurements for CO2 reduction using several different Cu electrodes whose oxidation states are controlled. The spectroscopic investigations demonstrate that a copper oxide electrode (Cu2O) has low activity against CO2 reduction on the basis of low-level detection of CO as an intermediate of CO2 reduction. On the other hand, other Cu electrodes possessing an OH layer on the Cu surface (Cu­(OH)2/Cu) and metallic Cu exhibit higher CO2 reduction activity with significantly greater detection of produced CO. When the metallic Cu electrode is used, only one peak (2060 cm–1) assignable to CO bound to the atop site of Cu is observed. However, additional peaks are detected in the range of 1900–2100 cm–1 when the Cu­(OH)2/Cu electrode is used. To understand these findings, the adsorption energy of CO on a Cu­(OH)2/Cu electrode and the CO stretching frequency were evaluated by performing DFT calculations. The adsorption energy is enhanced and the CO stretching frequencies are shifted to lower energy in comparison with those using a metallic Cu electrode. These results indicate that it is predominantly favorable to adsorb some CO molecules near the OH moiety of the Cu­(OH)2/Cu electrode and to induce interactions of CO molecules with each other. This observation is consistent with the results of controlled potential electrolysis (CPE), which generates C2+ products as previously reported. When CPE is carried out in D2O solution, residual and/or adsorbed OD– groups on Cu are detected by ATR-SEIRAS and the surface of the Cu­(OH)2/Cu electrode is confirmed to be metallic Cu, as measured by in situ Raman and XPS. From the ATR-SEIRAS experiments when switching from under CO2 to Ar during the electrochemical reduction, the OH layer is suggested to prevent deactivation of the Cu electrode via formation of the CO layer, which is detected as a bridge-bounded form on the metallic Cu electrode. The above findings indicate that the OH layer provides the advantage of attracting CO molecules closer to each other while reducing them to C2+ products without any degradation during electrolysis.

Cu electrode-based electrochemical CO 2 reduction using renewable energy is a promising method for conversion of CO 2 to useful compounds such as methane, ethylene, and ethanol. Heteroatom-doped and/or -derived Cu as oxide-derived Cu has been investigated in context of development of a stable catalyst with high selectivity, whereas the role of heteroatoms is not yet well understood. It is not known whether heteroatoms act as a moiety of the catalyst or simply induce reconstruction of the catalyst. This work is an investigation of the role of the heteroatom in electrocatalytic CO 2 reduction with a Cu electrode modified with methanethiol monolayers (MT−Cu), which is able to distinguish the presence of heteroatom contamination originating from electrolyte or air. Controlled potential electrolysis of CO 2 using an MT−Cu electrode at −1.8 V at Ag/AgCl exhibits greater selectivity for C 2 products than an unmodified polycrystalline Cu electrode (bare Cu). On the other hand, a sulfur-modified Cu (S−Cu) electrode predominantly generates formate as a CO 2 reduction product. In an investigation of the mechanism, an in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy instrument is used as a powerful surface analyzer. Scanning electron microscopy, grazing-incidence wideangle X-ray scattering (GIWAXS), and X-ray spectroscopy (XPS) are also employed in the investigation. The spectroscopic data show that reconstruction and formation of Cu + on the Cu surface occur at negative potential greater than −1.4 V vs Ag/AgCl by electrochemical reduction of methanethiol monolayers. DFT calculations are also performed under conditions close to the experimental conditions of electrical bias and aqueous electrolyte. The results indicate that a roughened surface is favorable for generating C 2 products. In addition, the Cu + moiety promotes generation of C 2 products, demonstrating that the doped heteroatom plays a crucial role in electrochemical CO 2 reduction.

The catalytic conversion of CO 2 to useful compounds is of great importance from the viewpoint of global warming and development of alternatives to fossil fuels. Electrochemical reduction of CO 2 using aromatic Nheterocylic molecules is a promising research area. We describe a high performance electrochemical system for reducing CO 2 to formate, methanol, and CO using imidazole incorporated into a phosphonium-type ionic liquid-modified Au electrode, imidazole@IL/Au, at a low onset-potential of −0.32 V versus Ag/AgCl. This represents a significant improvement relative to the onset-potential obtained using a conventional Au electrode (−0.56 V). In the reduction carried out at −0.4 V, formate is mainly generated and methanol and CO are also generated with high efficiency at −0.6 ∼ −0.8 V. The generation of methanol is confirmed by experiments using 13 CO 2 to generate 13 CH 3 OH. To understand the reaction behavior of CO 2 reduction, we characterized the reactions by conducting potential-and time-dependent in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements in D 2 O. During electrochemical CO 2 reduction at −0.8 V, the C−O stretching band for CDOD (or COD) increases and the CO stretching band for COOD increases at −0.4 V. These findings indicate that CO 2 reduction intermediates, CDOD (or COD) and COOD, are formed, depending on the reduction potential, to convert CO 2 to methanol and formate, respectively.

The mechanism of faradaic electro-swing for CO2 capture/release on a redox-active organic electrode is studied from the point of view of realizing a reversible process for CO2 separation. First, the cyclic voltammograms (CVs) of two redox-active organic monomers, anthraquinone (AQ) and 2,1,3-benzothiadiazole (BTZ), were measured under a CO2 atmosphere. The waveforms of CVs of the two redox-active organic monomers are altered under a CO2 atmosphere relative to a N2 atmosphere. There is a change in the number of redox waves from two to one for AQ and a change from reversible to irreversible waves for BTZ. To further understand the mechanisms of CO2 capture/release on redox-active organic compounds, redox-active polymer electrodes coated with polyanthraquinone (PAQ) and polybenzothiadiazole (PBTZ) were investigated using a spectroscopic analysis known as in situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) as well as density functional theory (DFT) calculations. The CVs measured with two redox-active polymer electrodes have more positive shifts of reduction potentials for PAQ and PBTZ under a CO2 atmosphere than under an N2 atmosphere, as measured versus Ag/AgNO3: −1.8 and −1.4 V → −1.4 and −0.9 V for PAQ and −2.5 V → −2.1 and −1.9 V for PBTZ. In addition, the oxidation current on the PBTZ electrode disappears only under a CO2 atmosphere. DFT calculations indicate that the positive shifts of reduction potentials in the two electrodes under CO2 conditions are due to an exergonic adsorption reaction of CO2 onto the redox-active organic compounds. To clarify the reaction behavior between CO2 and the redox-active organic electrode, an ATR–SEIRAS spectroscopic analysis was performed. Infrared peaks are observed at 2200 and 2100 cm–1 for PAQ and PBTZ electrodes, respectively, under a CO2 atmosphere, which have been confirmed by measurements under a 13CO2 atmosphere to have adsorbed CO2. The wavenumbers corresponding to the CO2 molecules adsorbed on the two electrodes are different. These findings indicate that one electron reduction and CO2 adsorption for each redox-active organic compound are occurring simultaneously. The calculated adsorption energy of CO2 for two redox-active organic electrodes indicates that the adsorption energies of two CO2 molecules for AQ and BTZ are −7.3 and −36.3 kJ/mol. The larger adsorption energy in BTZ than that in AQ is clearly related to the disappearance of the oxidation current. However, both adsorption energies indicating adsorption of CO2 onto organic units are less than the covalent bond energies of common organic compounds, indicating that such weak adsorptions are suitable for the faradaic electro-swing of CO2 capture/release on the redox-active organic units.